Electronic portal imaging devices (EPIDs) are known and traditionally used primarily in verification of patient setup and assessment of target and organ motion. With the advancement in digital imaging technologies, EPIDs have also developed into a tool for quality assurance of treatment machines and patient dosimetry.
Electronic portal imaging uses treatment beams having energy at megavolt (MV) levels to acquire images, and hence generally requires imagers having thick scintillators to effectively absorb x-rays. However, with increased thickness of scintillators, optical photons generated by absorbed x-rays may undergo extensive spreading or cross-talk, resulting in image blurring.
Pixelated scintillators, which can limit lateral spread of light photons, are developed. One conventional method of making pixelated scintillators uses crystalline scintillators such as cadmium tungstate (CdWO4 or CWO), cesium iodide (CsI), and bismuth germanate (Bi4Ge3O12 or BGO), etc. Crystalline scintillators are expensive. Crystalline scintillators are also difficult to process. For example, CWO has a cleave plane (010) that tends to break, chip or fracture during cutting, lapping or polishing. CsI is mildly hydroscopic, very soft, and susceptible to scratching and bending. Portal imagers built using this approach must minimize radiative cross talk to preserve spatial resolution. Radiative cross talk may be produced by scattering of Compton electrons between pixels, or off angle or scattered x-rays that transverse multiple pixels generating light in multiple pixels. Therefore, dense materials such as lead or tungsten typically are used to isolate pixels, resulting in very expensive imagers with a lower fill factor.
In another conventional method of making pixelated scintillators, scintillating glasses are built into scintillating fiber optic face plates (SFOPs) for mounting to a detector array. Pulling scintillating glasses into SFOPs may produce arrays of high resolution but at considerably increased costs. Moreover, there is significant light loss that can impede image quality. Some scintillating glasses may lose about 20 to 30 percent of their intrinsic scintillation light output when drawn into fiber optics. Furthermore, the geometry of a fiber optic may not be ideal for two reasons: the cladding reduces the fill factor and only light that is emitted within the critical angle for total internal reflection is collected and the numerical apertures of scintillating glasses tend to be low since the index of refraction of the core is similar to the index of refraction of the cladding. As a result, while these imagers are very high resolution, they may have reduced detective quantum efficiencies (DQEs) and may not produce enough light to be compatible with the readout electronics.
Another conventional method of making imagers uses gadolinium oxisulfide (GOS) screens with a copper buildup plate (Cu-GOS). GOS is ceramic and not optically clear. Therefore, GOS screens with copper buildup plates are limited in thickness. Because they are typically thin, most of x-rays having MeV energy levels simple pass through the imager. As a result, imagers using GOS screens with copper buildup plates generally have low DQEs.